LibGUI: Don't fire visibility-tracking timers in non-visible widgets

We were already avoiding firing timers within non-visible *windows*.
This patch extends the mechanism to support timers within non-visible
*widgets*.

Userland/Libraries/LibGUI/Widget.cpp

@@ -1114,4 +1114,9 @@ bool Widget::has_pending_drop() const
     return Application::the()->pending_drop_widget() == this;
 }
 
+bool Widget::is_visible_for_timer_purposes() const
+{
+    return is_visible();
+}
+
 }